Stoichiometric Calculations
The Art of Chemical Prediction: From Grams to Moles to Grams.
This tutorial combines everything we've learned so far.
← Go back to learn about Molar Mass.
The Problem: The Chemical Chef's Dilemma
Imagine you're a chemical chef. You have a balanced recipe (a chemical equation) and you know how to weigh your ingredients (molar mass). But now for the real question: if you have 10 grams of flour, how many grams of sugar do you need? And how many cookies can you make?
This is the essence of stoichiometry: using the relationships in a balanced equation to calculate the amounts of reactants and products. We can't just convert grams of one thing to grams of another. There's a crucial intermediate step.
Prerequisite: The Balanced Recipe
Before you can use the mole bridge, you must have a correctly balanced chemical equation. The coefficients in the balanced equation are the *only* source for the mole ratio. An unbalanced equation will give you the wrong answer, every time.
All examples in our calculator use pre-balanced equations. If you need a refresher on this crucial first step, please review our Balancing Equations Tutorial.
The Solution: The Mole Bridge
The only way to relate one substance to another in a chemical reaction is through the mole ratio provided by the coefficients in the balanced equation. You cannot convert directly from grams of substance A to grams of substance B.
You must first cross the "mole bridge":
- Convert the mass of your starting substance (A) into moles using its molar mass.
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Use the mole ratio from the balanced equation to find the moles of your target substance (B).
How to Build the Mole Ratio
The mole ratio is a fraction built from the coefficients in the balanced equation. The key is to arrange it so your starting units cancel out.
moles of A × coefficient of B (what you want) coefficient of A (what you have) = moles of B
Rule of thumb: The substance you want to find goes on top (numerator). The substance you are starting from goes on the bottom (denominator).
- Convert the moles of your target substance (B) back into mass using its molar mass.
Your mission is to use the interactive calculator below to see this process in action. Select a reaction from the dropdown menu and see how the calculations change. If we start with 10 grams of methane (CH₄), how many grams of water (H₂O) can we produce?
You Can Predict the Future! What's Next?
You now have the core skill of a chemist: predicting the outcome of a reaction. But what happens when you don't have the perfect ratio of ingredients? The next step is to learn about Limiting Reactants.